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Anyone travelling west from Auckland this Saturday should plan for delays, as State Highway 16 will be closed for 12 hours from Great North Rd to Patiki Rd.

The closure on the causeway section of the Northwestern Motorway, between 8pm on Saturday night and 10am on Sunday morning, is to allow a construction team to pour concrete to connect the existing bridge at the city end of the Causeway to a new parallel bridge.

"The concrete takes about 12 hours to harden and strengthen and, because vibrations from passing traffic can affect that process, all the westbound lanes need to be closed," said Brett Gliddon, the Transport Agency's Auckland highway manager.

Motorists will be directed along a detour route around Great North Rd, Rosebank Rd and Patiki Rd before rejoining the motorway.

All lanes heading towards the city will stay open throughout the weekend, along with the Rosebank Rd on-ramp, and the Northwestern Cycleway.

Those heading towards Te Atatu are advised to continue down Ash Rd, rather than rejoin the motorway at Patiki Rd.

This work is part of the bigger project of completing the $2.4 billion Western Ring Route, one of seven roads the government is prioritising to support economic growth. It focuses on improving safety and upgrading provisions for pedestrians, cyclists and buses.

Once the route has been completed it will be an alternative to State Highway 1, improving network resilience, travel time reliability and bus shoulder lanes.

This weekend there will also be changes for motorists planning to use State Highway 20A in Mangere.

City-bound lanes will change position on Sunday until the middle of 2017, to create space for the construction of a trench under the Kirkbride Rd intersection.

Those heading to the airport are warned to factor in an extra five minutes.

The temporary layout will involve some turning changes, including no right turn into Kirkbride Rd when heading to the city from the airport.